1. Field of the Invention
The present invention relates to a power converter and, more particularly, to a power converter including a control system in consideration of a countermeasure against instantaneous power interruption.
2. Description of the Related Art
FIG. 7 shows a typical power converter for supplying an AC power of a variable voltage and a variable frequency to an AC motor and variably controlling the motor speed.
This self-excited converter is controlled as follows.
Speed reference signal E7 supplied from speed setting circuit 7 is separated into output voltage reference V* and output frequency reference f* of self-exited inverters 2 and 4 by V/F reference generator 8, and the references are input to PWM controller 9. The pulse width control is performed in PWM controller 9 in accordance with supplied output voltage reference V* and output frequency reference f* and ON/OFF patterns of switching devices (not shown) of inverter 4 are generated. The ON/OFF patterns are respectively converted into ON/OFF gate signals by gate signal generator 10, and the signals are output to each switching device (not shown) as gate signals.
With the above arrangement, predetermined voltage and frequency are supplied to load motor 5, and the speed of load motor 5 is variably controlled.
Power interruption detector 6 is arranged at AC power supply 1 to detect power interruption when the power of the AC power supply is suspended. When power interruption detector 6 detects power interruption, an operation of gate signal generator 10 is stopped in response to detection signal E6 so that an ON/OFF operation of the switching device (not shown) in inverter 4 is stopped. Therefore, the self-excited inverters are stopped and wait until power is restored.
When the power of AC power supply 1 is restored and power interruption detector 6 detects this state, the self-excited inverters are restarted after a predetermined time period is elapsed, i.e., terminal voltage Vac of load motor 5 is set at a predetermined value or less. This is to prevent an overcurrent of outputs from self-excited inverters 2 and 4.
The power converter having the above arrangement has the following problems.
More specifically, the self-excited inverters cannot be restarted until terminal voltage Vac of load motor 5 is set at a predetermined value or less. In addition, since an output is gradually increased so as to prevent the overcurrent, a sufficient time period is required until the self-excited inverters are restarted to generate a predetermined torque, and a reduction in motor speed is stopped to reaccelerate the motor. Therefore, the motor speed is undesirably reduced more than necessary.
FIG. 8 shows another conventional arrangement wherein a speed of an AC motor is variably controlled using a power converter such as an inverter.
Referring to FIG. 8, when AC input voltage Vi fails due to power interruption, the failure is detected by power interruption detector 180. A gate signal of rectifier 2 is shifted or shift-blocked by phase controller 210, and inverters 2 and 4 are stopped. Thereafter, the power restoration of power supply 1 is detected by power interruption detector 180. Then, the operation is restarted after it is confirmed that terminal voltage Vac of load motor 5 is set at a predetermined value or less. This is to prevent the overcurrent of inverters 2 and 4 by voltage controller 200 and oscillator 220. Note that stop circuit 240 monitors an output from current transformer 14. When an overcurrent is supplied to load motor 5, the operation of inverters 2 and 4 is stopped.
In the above-mentioned power converter, in order to assure an output voltage during a power interruption period, the capacitance of DC smoothing circuit 130 must be increased. In this case, the power converter stops control upon detection of power interruption, and is restarted after power restoration. In such a converter, when motor terminal voltage Vac remains, an overcurrent tends to occur in accordance with a motor terminal voltage phase, power converter output phase, and a DC voltage Vdc level. For this reason, upon restart of the power converter, after motor terminal voltage Vac is set at a predetermined value or less, the output frequency must be gradually increased while the output voltage and frequency of inverter 4 are precisely controlled and overcurrent is suppressed. Then, a time required for restart is prolonged, and the motor speed is undesirably reduced more than necessary.